Misfires above 4,500

[jon stead]

Stumped again.

Engine still ticks over nice and rides nice at low revs but doesn't pull above 4500 rpm. Number 3 plug was sooty, so I put the original carbs back on, hard to say if better or not, but plug 3 still sooting  up. So unlikely the same fault is with both sets of carbs.

Checked Boyer Branson electronic ignition advance is ok, I.e. Full advance at 4,500 rpm.

There's a new set of D7 plugs in, gapped correctly. Valve timing is correct and valve clearances are correct.

Now looking at electrics again. Confirmed plug cap resistance is correct and have also exchanged cap 3 and 2 for each other. If number 2 plug soots up then it's the plug cap at fault. Will check tomorrow. Also going to check all electronic ignition wiring tomorrow, coils and ht leads.

Edit: Corrected cylinder numbers - sorry, Chris

[kevski]

Do you have both resistor caps and plugs, if so that would most likely be the problem.

[JamesH]

Jon - stupid question but check your main jets. I had an almost identical issue on a 750 recently. Previous owner had drilled out the main jets to prob 150-160 (should have been running 120's) and it was the last thing I thought to check (the mains were stamped 120). May be a red herring but worth looking at when you have the carbs off...

[JamesH]

Ignore the above - didn't see that you'd tried two sets of carbs with same symptoms...

[jon stead]

Do you have both resistor caps and plugs, if so that would most likely be the problem.

Thanks. It's got 5k ohm resistor caps and non resistive plugs

[jon stead]

Jon - stupid question but check your main jets. I had an almost identical issue on a 750 recently. Previous owner had drilled out the main jets to prob 150-160 (should have been running 120's) and it was the last thing I thought to check (the mains were stamped 120). May be a red herring but worth looking at when you have the carbs off...

Thanks. Jets are correct in both sets of carbs I've got. Fault stays the same, whichever set if carbs I've got on.

[jon stead]

It's all stop again :-(

Checking the plugs to see if the fouling had moved to number 2 after swapping the plug caps yesterday, and I've gone and stripped the threads on number 2 cylinder. It's always been stiff to screw in and out but now it's given up the ghost. Sigh.

Question: can a helicoil repair be carried out in situ?

[Ashdowner]

Yes someone I know just had it done. It's not a helicoil but a solid insert, and it's done very carefully so no crap ends up in the barrel..  dougie@thethreadmaster.co.uk  07775510166 is a guy in Hampshire who did it for him in Sussex, but there will be someone local to you who can do it.

[Nurse Julie]

This thread may help
http://www.sohc.co.uk/index.php/topic,13481.msg108229.html#msg108229

[Rob62]

Be careful, its going to be very difficult if not impossible to avoid getting swarfe in the cylinder when fitting any kind of insert. Will need to be vacuumed out somehow.....

[paul G]

Be careful, its going to be very difficult if not impossible to avoid getting swarfe in the cylinder when fitting any kind of insert. Will need to be vacuumed out somehow.....

As far as I can remember the guy heavily greased the tap.
Removed the exhaust header and opened the exhaust valve.
At the same time he did the tapping he was blowing air into the head via the exhaust port!

[kevski]

Best quick fix is timesert not cheap, better fix is to have an ally insert made up, my Z1000 has all four done, and were done by a chap on the Z1 owners club who goes by the name of buzzard, even more expensive but beautifully done.

[Nurse Julie]

Be careful, its going to be very difficult if not impossible to avoid getting swarfe in the cylinder when fitting any kind of insert. Will need to be vacuumed out somehow.....

As far as I can remember the guy heavily greased the tap.
Removed the exhaust header and opened the exhaust valve.
At the same time he did the tapping he was blowing air into the head via the exhaust port!

Yes, that was the one I posted the link to above.

[jon stead]

Thanks all for your input.

I've ordered my kit up, insert rather than helicoil, and I already knew about using grease to catch the swarf. I like the idea of blowing air through the open exhaust valve. I was just going to fashion a thin tube on the end of my Henry vacuum cleaner and have a good suck around inside the cylinder before firing up.

[Tomb]

We use Demi-John rubber bungs in the exhaust, drilled for a 6mm air pipe to shove in.

https://www.homebrewcentre.co.uk/demijohn-solid-rubber-bung?gclid=CjwKCAjw0qLOBRBUEiwAMG5xMP5pIvm2OhcZyPo22W-0ttFRXA7E-yym9FFghhEiilg7_44bhWLPkRoCTZkQAvD_BwE

HTH